1. Field of the Invention
The invention generally relates to a cathode for emitting electrons. More particularly, a dispenser cathode having at least two electron emitting surfaces oriented in different directions.
2. Background of the Invention
A combination of an electron source and energized electrode grid generates ionizing electrons that may be used in conventional hot cathode based ion sources employed in radiation generators, such as neutron generators or as a source for electrons in x-ray-producing accelerators. These devices are typically uni-directional whereby the source of electrons is at one end of an acceleration column and a target from which emanates the desired radiation, for example neutrons or x-rays, is at the opposing end. There are several possible sources of free energetic electrons for ion sources. The most common source is via the thermionic process in which when a metallic surface is heated, electrons are freed with thermal energies. The simplest source of thermionic electrons is a heated tungsten filament. By passing a current through the filament, ohmic heating occurs and electrons are released. When a biasing voltage is applied to the filament, the freed electrons can accelerate into a nearby volume. Such a simple filament poses some critical issues including: the source of electrons is distributed in space along the surface of the heating filament (i.e., not a point source) and is dependent on the temperature and localized accelerating (extracting) field; and the filament is susceptible to ion bombardment and thus sputtering, limiting its useful life.
A dispenser or hot cathode mitigates most of the drawbacks of a filament by providing a planar surface from which electrons are emitted. A typical dispenser cathode includes a tubular body containing a heater coil embedded in a ceramic matrix. The container or can may have any desired cross-section, such as square or round. At one end of the tube is a disk of an emitter material, typically porous tungsten with a work function lowered by a suitable doping process. At the other end of the tube is the ceramic matrix with outwardly extending leads. The robustness of the dispenser cathode design is achieved at the expense of the total operating power when compared to a simple filament. Indeed, the added thermal mass of the body, though, makes for a more uniform temperature of the emitter surface, therefore results in more uniform electron emission, thus requiring greater heating power. The thermionically emitted electrons can be accelerated into a beam by creating an electrostatic field, such as an electrode, for example a grid, in front of the emitting surface.
A dispenser cathode is disclosed in U.S. Pat. No. 4,823,044 to Falce. The dispenser cathode includes a cup shaped reservoir containing a pellet that is a porous mixture of tungsten doped with barium calcium aluminate. An outward facing surface of the pellet is sealed with a porous sintered tungsten plug. A resistance coil located adjacent the reservoir provides heat to effect emission of electrons.
A common feature of most conventional radiation generators is uni-directional particle acceleration. Such basic particle accelerators include, at a minimum, a source of charged particles, an acceleration column for transport of charged particles and a target. In the case of neutron and x-ray generators, the accelerated particles are made to collide with the target which becomes the source of radiation. In some specific configurations of accelerating fields, it is of interest to make use of two or more directions dictated by either or both the physical geometry and the orientation of the accelerating field. For these instances, the sources of charged particles provide charges which are acceptable by each and all directions of the accelerating field. For example, U.S. Pat. No. 4,577,156 to Kerst discloses two Betatron tubes, one above the other, and each tube having a separate electron injector and target. A first injector injects a beam of electrons into the first tube in a first direction when an accelerating flux is changing from its positive maximum to its negative maximum. The second injector then injects a beam of electrons into the second tube in an opposing second direction when the accelerating flux is changing from its negative maximum to its positive maximum. A single tube embodiment having two injectors spaced apart in the same tube is also disclosed.
The U.S. Pat. No. 7,148,613 to Dally, et al. discloses a thermionic emission cathode having circumferential emitters surrounding a central heater such that the cathode emits electrons in up to 360° about the central heater. An electron impervious shield surrounds the cathode and has windows that enable collimated emission of electrons in desired directions.
For applications such as a pulsed Betatron, there remains a need for a common source of electrons that can be provided in controlled bursts in multiple directions. Such a device could improve the efficiency of the Betatron. For applications such as a multiple point source grounded target neutron generator, there remains a need for a common source of electrons that can be provided in controlled bursts in multiple directions. Such a device could extend the measurement capability of the neutron generator.